This invention relates to a rotor of a flywheel magneto and a manufacturing method thereof, and more particularly to a construction and method for joining a boss mounted on a driving shaft to a flywheel through metal flow of a joining member.
A flywheel magneto driven by an engine comprises a cup-shaped flywheel mounting thereon a plurality of permanent magnets, and a boss for joining the cup-shaped flywheel to a driving shaft of the engine. The boss has a flange overlapped on the cup-shaped flywheel. The overlapped flange and the cup-shaped flywheel are assembled by a plurality of rivets to form a rotor. In a space defined by the flywheel and the boss, a plurality of iron cors and coils are disposed. The cup-shaped flywheel is composed of heavy metal members because of moderating by its inertia any fluctuation of speed or torque in the engine. For the rotors of this kind of flywheel magnetos, it is necessary to satisfy the following conditions from the design point:
(a) not to involve any problem at a normal rotational frequency of 11,000 r.p.m,
(b) not to break at 22,000 r.p.m, and
(c) to endure a head variations of from 40.degree. C. to 180.degree. C. In consideration of the various conditions, the boss and the flywheel are fixed with 6.about.9 rivets which each are made of carbon steel and has a diameter of 4.about.6 mm. In order to satisfy the requirements of strength of the assembled rotor, a high dimentional precision in pitches between holes for the rivets is required. An error in measurement of the pitch in making the holes causes concentration of stresses in the rivets, which may results in loosing the joint portion of the rotor. Therefore, there is the problem of the costly production of such precise parts.
For providing a small size of the flywheel magneto, it is desirable to reduce its axial size. In this construction of the flywheel magneto, however, the axial size can not be reduced because the flange of the boss and the flywheel are overlapped, and the heads of the rivets projected into the inner space of the flywheel magneto are necessary to be spaced enoughly from the coil disposed in the inner space not to interfere with the coil during the rotation of the rotor, as shown in U.S. Pat. No. 3,783,315. It is difficult to reduce the thickness of the flywheel or the flange because the thinner can not endure a large load caused by the inertia due to the rotation of the rotor.
On the other hand, as joining of two members, force fit and welding are well-known. In the force fit, an error in measurement in making the members to be joined influences greatly joining strength, which is limited. Therefore, the force fit can not be applied for assembling the flywheel and the boss. The welding accompanys thermal deformation of a member to be joined because of being heated. Accordingly, it is difficult to manufacture the above-mentioned rotor with the desired accuracy by the welding.
Further, there is a method of joining two members by plastic deformation or metal flow of joining material of metal inserted between the members. Such a method is disclosed, for example in U.S. Pat. No. 3,559,946, wherein a groove having a rectangular section is formed on each face to be joined of two member, and a joining member of metal in filled in the groove through the metal flow. The joining material is not fully filled in the groove because the joining material does not flow rectangularly or along the surface of the rectangular groove. Therefore, there is a gap between the joining material filled and the surface of the groove so that sufficiently strong joining can not be obtained. Further, the joining material disclosed in the U.S. patent is filled only in a limited portion of the member to be joined, which means that the joining material is not utilized sufficiently. Accordingly, sufficient joining strength can not be obtained.